mm: multi-gen LRU: improve design doc

This patch improves the design doc. Specifically,
  1. add a section for the per-memcg mm_struct list, and
  2. add a section for the PID controller.

Link: https://lkml.kernel.org/r/20230214035445.1250139-2-talumbau@google.com
Signed-off-by: T.J. Alumbaugh <talumbau@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>